Rotating components in large machines such as gas turbines, steam turbines, or aircraft engines, operate under large centrifugal stresses and may suffer breaks or cracks located at various points in the body of the rotating component. For example, engines have rotating compressor units that move and compress air. The compressor has blades around the periphery of a rotating disk, and the high speeds that the blades rotate induce high levels of stresses that tend to pull the components apart and cause cracks. In addition, through problems of excessive wear, the rotating assembly components may become unbalanced, which imposes unacceptable loads on the unit, possibly resulting in the creation of cracks. Once a crack is formed, it may grow in size until the rotating component is separated from the main assembly, thus resulting in a catastrophic failure. The problem is of gravest importance with passenger-carrying jet airliner engines, where upon such failure, separated blade components penetrate the engine compartment and/or the aircraft itself, thus impairing the air worthiness of the aircraft.
The most common method of detecting anomalies, such as cracks, in a rotating component of an engine, is to electronically monitor vibrations emanating from the rotating component. The idea is that a rotating assembly that lacks any cracks or similar imperfections is optimally balanced during the manufacturing and testing process, therefore vibrations due to asymmetrical rotational forces are minimal. Monitoring of dangerously high vibration amplitudes, or at least, large incremental changes over otherwise normal patterns of vibration, provides a good indication of a break or excessive wear present anywhere in the rotating unit.
Sonnichsen and Milatovic (U.S. Pat. No. 6,456,945) discloses a method of detecting cracks in a rotor, by measuring the rotational speed and vibration of the rotor. A processor subtracts a baseline vibration signal from the detected vibration measurement to produce a vibration difference signal. Subsequently, the amplitude and the phase of the vibration difference signal is measured to determine if anomaly, such as a crack, has occurred. Similarly, Gasch and Mingfu (U.S. Pat. No. 5,533,400), discloses a method for sensing cracks formed in rotating shafts, wherein the harmonic vibration components of flexural vibrations of the shaft are measured.
The vibration sensing method even though it offers the advantage of real time detection of anomalies in the body of the rotating unit, lacks sensitivity to the exact or even the approximate location of the crack. Since the method is based on sensing the vibration caused by unbalance during the operation of the unit, and said unbalance is created by the presence of cracks anywhere in the body of the unit, which produce an aggregate result, the method can not be tailored to sense small individual cracks in high-risk areas, such as the tip of the blade. Furthermore, the vibration sensing method is relatively complex, as it requires several peripheral electronic components, such as filters, speed sensors, etc.
In addition to the above real time monitoring of the occurrence of cracks in rotating units in engines, manual inspection may also be performed. More specifically, the engine is taken apart and the unit under inspection is immersed in a fluorescent solution. The fluorescent material will penetrate any cracks in the area and make them visible under ultraviolet light. Manual inspections are costly and time consuming, since the engine must be taken out of service and be disassembled.